Field of the Invention
This invention relates to a low shrinking unsaturated polyester resin composition which possesses excellent dispersion stability on storage and which comprises an unsaturated polyester resin and a block copolymer.
Sheet molding compositions (hereinafter designated as SMCs) produced by blending unsaturated polyester resins with filler, pigment, thickening agent, fibrous reinforcement, hardening agent, stabilizer, mold releasing agent, bulk molding compounds and the like have been used in increasing amounts as new industrial materials for use in the field of machine molding. These compositions fulfill the requests for materials capable of reducing the labor cost and improving the productivity and the working environment in the reinforced plastic industries.
Attempts have been made to decrease the volume shrinkage of unsaturated polyester resins on hardening by blending the unsaturated polyester resins with thermoplastic resins such as polystyrene, poly(methylmethacrylate) and the like, as a low shrinkage agent. Application of the thus obtained low shrinking unsaturated polyester resin compositions as industrial materials in the field of machine molding should greatly contribute to improve this class of molding materials, in a high degree, because the resulting molded products can be considerably improved in their appearance and dimensional accuracy.
A conventional low shrinking resin composition prepared by blending low shrinking agents with the unsaturated polyester resins, together with filler, pigment, thickening agent, fibrous reinforcement, hardening agent, stabilizer, mold releasing agent and the like, can give molded products having excellent gloss and surface smoothness when molding is performed soon after blending of the composition.
However, the conventional low shrinking resin composition has the drawback that the low shrinking agent, which initially exists, in a dispersed state therein, gradually fails to maintain the dispersed state during storage and results in spoilage of the dispersion stability, because of the migration of polymerizable monomers dissolved in the low shrinking agent. Thus, the composition loses the ability of imparting an excellent gloss and smoothness to the surface of the hardened molded products.
As a process for improving the dispersion stability of such a low shrinking unsaturated polyester resin composition, a process, for example, which is described in Japanese Patent Laid-Open Publication No.SHO 52-21092 (1977), has been proposed, wherein a graft copolymer composition having a fine gel structure is blended, as the low shrinking agent, with the unsaturated polyester resin for improving the dispersion stability on storage. The graft copolymer composition having the fine gel-structure is prepared by graft-copolymerizing an emulsion of gum material with a styrene type monomer and a vinyl type monomer which is copolymerizable with a small quantity of methacrylic acid ester and/or acrylic acid ester.
It is true that the graft copolymer composition having the fine gel-structure of the process is excellent in dispersion stability when it is blended with the unsaturated polyester resin, as compared with conventional low shrinking agents, such as polystyrene and polymethylmethacrylate. However, the composition is not sufficient enough in the dispersion stability, since it contains a lesser amount of the graft copolymer and a larger amount of homopolymers of the above mentioned monomers because of the low efficiency of the graft-copolymerization.